The present invention relates to a device for providing electrical continuity between two electrically conductive surfaces.
Enclosures for electrical and electronic equipment are frequently required to function as an electromagnetic shield to minimize the undesirable passage of electromagnetic fields into and out of the enclosure. The shielding capability of an enclosure is usually quantitatively defined by the amount by which it will attenuate electromagnetic fields which tend to enter or exit the enclosure. The attentuation is conventionally expressed as the ratio of the incident to attenuated field levels, in decibels, and is termed "Shielding Effectiveness". For high shielding effectiveness the enclosures are frequently constructed of metal having a high electrical conductivity.
As a practical matter the intrinsic shielding effectiveness of the material of the enclosure is generally of less concern than leakage through seams (see Henry Ott, Noise Reduction Techniques in Electronic Systems, Pub. John Wiley & Sons, 1976, p. 164). To minimize the seam leakage, particular care is required to maintain high, essentially continuous, electrical conductivity across the seams. For permanent seams this is ideally achieved by means of continuous welding or brazing (see Keiser, Principles of Electromagnetic Compatibility, Pub. Artech House, Inc., 1979, p. 121). Non-permanent seams around access covers, doors and the like, present a special problem. If a direct metal-to-metal contact could be obtained all along the length of the seam, then leakage from the seam would be negligible (see Quine et al, Electromagnetic Shielding Principles, Vol. 1, Rensselaer Polytechnic Institute, Mar. 1, 1956, p. 82, 83). In practice, with only a discrete number of fasteners and the inevitable unevenness in manufactured parts, there will be slit-type openings in the seam between the fasteners. Leakage through these openings can be significant, especially at high frequencies (see Keiser, ibid, p. 120).
Slit-type leakage can be reduced by minimizing the lengths of the slits by means of increasing the number of fasteners. This is often impractical, or makes for an unattractive product, and alternative approaches are used.
For achieving a multiplicity of electrically conductive contacts, devices known as gaskets are used. Especially effective types of gaskets are spring finger strips, as produced, for example, by Instrument Specialties Co., Inc. of Delaware Water Gap, PA or Tech-Etch, Inc. of Plymouth, MA. The spring fingers are designed to have a range of flexibility, or compliance, which is adequate to comply with unevenness in the manufactured parts.
The known finger strips are not completely satisfactory. The manufacturer frequently specifies performance of the product over a compression range which limits maximum compression to prevent permanent set in, or complete rupture of, the gasket (see Tech Etch, Inc., Standard Product Specification-Finger Strips-RFI/EMI Shielding or Grounding; see also Instrument Specialties, Inc., Product Specification Sheet for Sticky Fingers for Instant RFI/EMI Shielding, U.S. Pat. No. 3,504,095). This application limitation is undesirable, as auxiliary means required to limit the compression are frequently costly. Further, experimental results by the inventor, Richard Mohr, with a gasket he specifically designed to be compressed in use beyond the typically recommended range of 25% to 75%, and to be completely flattened without permanent set, showed shielding performance improvement, 100 KHz to 300 MHz, as follows:
______________________________________ MEASURED TRANSFER IMPEDANCE VS. PERCENT COMPRESSION Compression Relative Improvement (%) (dB) ______________________________________ 10 0 (Baseline Reference) 25 12 50 22 75 27 100 31 ______________________________________
In the gasket tested, when closing force was increased about 35% beyond that required for 100% compression, shielding performance increased an additional 4 dB.
An additional disadvantage of known spring finger gaskets is that they are prone to damage, as by snagging. Known means to minimize the possibility of such snagging damage typically add to the cost of the device or the cost of their utilization, and/or inhibit their electrical performance. The foregoing disadvantages will become clear in the following review of the existing state of the art.
Auxiliary means of preventing over-compression include the use of a groove in one of the conducting surfaces of the seam to house the gasket as disclosed for example in U.S. Pat. No. 4,572,921. Similar solutions are employed in Soviet Inventor's Certificates Nos. 438,156 and 372,759. While the approach is effective, machining the groove is costly.
Another approach for preventing over-compression includes the use of screws, rivets or special fasteners to secure the spring finger strip in place and to double as a positive stop, such as suggested, for example, by Instrument Specialties Co., Inc. for their Part Number 97-438, and as disclosed in German Patent Document No. 1,035,219. These stops are however not fully positive since the fasteners for securing the seam must be physically offset from the fasteners which secure the spring finger strip. The seam therefore tends to buckle as it is secured.
In the device disclosed in the U.S. Pat. No. 3,962,550 there is no complete closure since the gap is still left. The same disadvantage is present in the devices disclosed in the Danish Patent No. 93,744 and German Patents Nos. 1,035,219 and 2,235,216. In the U.S. Pat. No. 4,623,752 the spring is not designed to be fully compressed; maximum compression is limited by push-in fasteners which can function as a positive stop. In European Patent No. 0 030 529 a hinge is provided which could be employed to limit compression, however hinges are known to provide only poor conductive paths in a shield (see Filtron Company, Inc., Interference Reduction Guide for Design Engineers, Vol. 1, U.S. Dept. of Commerce, National Technical Information Service, Aug., 1964, p. 2-31). In the European Patent No. 0 159 407 an extra pocket is provided in one member of the seam to house opposing sets of spring fingers, and the opposing member of the seam enters the pocket displacing the spring fingers laterally. The disadvantages of this arrangement are, the added complexity due to the pocket and the two sets of fingers, and the incomplete compression of the spring fingers. A similar arrangement, and similar disadvantages exist, in the German Patent No. 26 01 277.
The possibility of snagging is minimized in the Soviet Inventor's Certificate No. 372759 but with the added complication of a required containment groove. The arrangements in European Patent No. 0 159 407 and German Patent No. 26 01 277 protect the fingers but with added mechanical complexity, and with degraded electrical performance because of incomplete compression.
The snagging is minimized in No-Snag Fingers manufactured by Tech-Etch, Inc., Part No. 187M60 (see Tech Etch, Inc., product specification sheet, No-Snag Fingers Part No. 187M60, NSFIA). In these strips the fingers are joined at both ends and the joining strips are rolled under to prevent snagging. The joining of fingers at both ends limits however their independent action to the detriment of the electrical seal. In the Foldover series by Instrument Specialties Co., Inc. Part 97-452, additional spring fingers in the base section of the strip are folded under the strip and up and over the tips of the spring fingers (see Instrument Specialties, Co., Inc., Product Specification Sheet for Sticky Fingers, U.S. Pat. No. 3,504,095, for Foldover Series). This protects the fingers, however the finger tips no longer directly contact the surfaces of the seam, to the detriment of the electrical seal.